<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    console.log(typeof(x))
    //undefined
    var x;
    if(x === null)//三个等号和两个等号，有什么区别
            console.log("是一回事")
    else
            console.log("不一样")
    x="a";
    console.log(typeof(x))
    x=1;
    console.log(typeof(x))
    x={}
    console.log(typeof(x))

</script>

</body>
</html>